Investigations on organic electroluminescent devices using organic materials as electron hole transport materials and electric charge (hereafter simply referred to as charge) transport materials constituting electroluminescent devices have been actively pursued in recent years.
As conventional charge transport materials, anthracene derivatives, anthraquinone derivatives, imidazole derivatives, styryl derivatives, hydrazine derivatives, triphenylamine compounds, poly(N-vinylcarbazole), oxadiazole, and other compounds are known.
Liquid crystalline compounds are applied, as display materials, to various apparatus such as clocks, electronic desk-top calculators, television sets, personal computers, and cellular phones. Liquid crystalline substances are categorized as thermotropic liquid crystals and lyotropic liquid crystals based on their phase transition means. From the viewpoint of molecular alignment, these liquid crystals are categorized into three groups, namely smectic liquid crystals, nematic liquid crystals, and cholesteric liquid crystals. The liquid crystals have a synonym of anisotropic liquids and are optically anisotropic as in optically uniaxial crystals. Orthoscopic observation is observation performed between regular crossed nicols and is useful for the identification of the types of liquid crystals or for the determination of transition temperatures of liquid crystal phases. The individual liquid crystals show characteristic birefringent optical patterns upon the orthoscopic observation, and the smectic crystals are further categorized as A, B, C, D, E, F, G and other smectic phases.
Hanna et al. have found liquid crystalline compounds having a smectic liquid crystal phase are capable of transporting charges and have proposed charge transport materials using these liquid crystalline compounds. They have proposed, for example, a liquid crystalline charge transport material exhibiting smectic liquid crystallinity and having a reduction potential with respect to a standard calomel electrode (SCE) in the range of from −0.3 to −0.6 (V vs. SCE) (Japanese Patent Laid open No. 09-316442), a liquid crystalline charge transport material comprising a liquid crystalline compound exhibiting a smectic phase having self-orientation properties and a predetermined amount of fullerene C70 capable of sensitizing (Japanese Patent Laid open No. 11-162648), a high polymer membrane comprising a liquid crystalline charge transport material dispersed therein in which a liquid crystalline compound exhibiting a smectic phase is dispersed in an organic polymeric matrix (Japanese Patent Laid open No. 11-172118), a liquid crystalline charge transport material comprising a mixture containing a smectic liquid crystalline compound (Japanese Patent Laid open No. 11-199871), a liquid crystalline charge transport material having smectic liquid crystallinity and an electron mobility or electron hole mobility of not less than 1×10−5 cm2/v.s (Japanese Patent Laid open No. 10-312711), and a liquid crystalline charge transport material comprising a smectic liquid crystalline compound having, in one molecule, a functional group capable of forming a new intermolecular or intramolecular bond and a functional group capable of transporting holes and/or electron charges (Japanese Unexamined Patent Application Publication No. 11-209761).
The above-proposed smectic liquid crystalline compounds are smectic liquid crystalline compounds having 6 π-electron aromatic rings such as benzene ring, pyridine ring, pyrimidine ring, pyridazine ring, pyrazine ring, tropolone ring, and compounds having 10 π-electron aromatic rings such as naphthalene ring, azulene ring, benzofuran ring, indole ring, indazole ring, benzothiazole ring, benzoxazole ring, benzimidazole ring, quinoline ring, isoquinoline ring, quinazoline ring, quinoxaline ring, and compounds having 14 π-electron aromatic rings such as phenanthrene ring, anthracene ring, and others. These compounds are used for charge transporting in a smectic A liquid crystal phase. Such charge transporting with the aid of smectic A phase is charge transporting using spread of a conjugated system in the molecule. Accordingly, none of these compounds can exhibit excellent charge transporting capability unless they are excited by, for example, light. In addition, they have a low current density, at most on the order of nanoamperes per square centimeter.